The existence of the curtaining effect in an extruded thermoplastic layer is a problem that has long needed a solution. FIG. 1 depicts a two layer laminate of thermoplastic materials, in which the curtaining effect is shown to be present in both layers. As explained in detail below, the convergence of molten thermoplastic streams at substantially equal flow velocities in an extrusion die is known. However, a problem is that external adjustment of the die is required. The present invention is concerned with overcoming these problems in a multimanifold extrusion die.
As exemplified by U.S. Pat. No. 3,877,857 to Melead, multiple melt chamber extrusion dies are known. This type of die has two die halves between which a center divider extends, and has in each die half an upstream and downstream melt or manifold chamber connected by a communicating channel having a narrow cross-sectional area. A molten stream flows from the downstream manifold chamber through a second channel having a narrow cross-sectional area, and then converges with another molten stream to form a melt-laminate.
Also known, as illustrated by U.S. Pat. No. 3,694,119 to Scheibling, is an extrusion nozzle having a central tongue separating two flow passages that terminate at a discharge slot. In a special embodiment of the extrusion nozzle, the feed channels for the molten thermoplastic materials and/or the longitudinal slits connected to these channels are of such a construction that their cross-section is reduced toward the center of the channels and/or slits, and it is stated that this causes an improvement of the distribution of pressure in the material issuing from the extruders, so that very uniform layers are produced. In one embodiment of the nozzle, a molten thermoplastic stream flows through a pair of distribution channels joined by a narrow flow passage channel.
As exemplified by U.S. Pat. No. 4,344,907 to Herrington, a co-axial tubular extrusion die is known that has a flow restriction in the flow path of the lower viscosity resin to increase the pressure drop of the resin as it passes through the die. Separating the two flow paths of this die is a divider having a wall that in part forms the flow restriction.
Also known, as illustrated by FIG. 3 of my U.S. Pat. Nos. 4,152,387 and 4,197,069, is a multimanifold coextrusion die having an adjustable divider provided between any two of the flow channels thereof. Each flow channel includes a back pressure cavity and a flow restriction channel located between the back pressure cavity and the point of convergence of the flow channels. This coextrusion die provides for adjustment of flow restriction channel width so as to cause the converging molten thermoplastic streams to converge at substantially equal flow velocities, by manual manipulation of the adjustable divider. As a result, this die promotes laminar flow at the point of convergence. However, a drawback of this multimanifold extrusion die is that the layers of a laminate produced thereby, exhibit the curtaining effect. Furthermore, even though the convergence of melt streams is effected at substantially equal flow velocities, external adjustment of the divider is required. Hence, there is a need for a multimanifold extrusion die that minimizes or eliminates the curtaining effect, in addition to retaining the advances in the art provided by the prior art die of FIG. 3 of U.S. Pat. Nos. 4,152,387 and 4,197,069. Such an improved die would be especially remarkable if it were capable of overcoming the curtaining effect for resins of varying viscosities and hence varying flow rates, merely by removal of and replacement of a component thereof with an interchangeable component precisely configured for a specific resin viscosity. Moreover, there is a need for a multimanifold extrusion die that in addition automatically provides for convergence of molten thermoplastic streams at substantially equal flow velocities. Such a die would make possible an improved process for melt-lamination of thermoplastic materials.